Although the idea of making your own calculator may seem redundant at first, doing so is an interesting journey in logic and the fundamentals of how calculators can work. This has been documented well by Don van der Drift who uses a simple numeric keypad, Arduino and LCD module to create a four-function integer calculator.
An interesting aside in this project is the explanation of how the numeric keypad is connected to the Arduino - normally this requires seven I/O pins, however Don has used a method of using resistors with the keypad matrix that allows all buttons to be detected with only one analogue input pin, which is very useful.
If you've been looking for an approachable, easy to read book to learn about Arduino, or as a gift for someone else - you can't go past "Arduino Workshop - A Hands-On Introduction with 65 Projects” by John Boxall.
Arduino Workshop takes the reader from having zero knowledge about the Arduino platform, electronics and programming and leaves them with the know-how and instructions on everything from blinking an LED, to robotics, wireless data, cellular communications, Internet connected systems and more. For more information including a sample chapter and table of contents, visit the book page.